The invention relates to an apparatus for checking weights per unit area during production of sheets of material by means of a source of radiation, which irradiates the sheet of material or the material being measured, and for detecting residual radiation on a side of the material being measured opposite to a radiation source using a gas-filled ionization detector.
Such apparatuses are used, for example, for continuously measuring the thickness of cold-rolled or hot-rolled metal sheets or foils and also for continuously measuring the thickness of paper sheets or the like. The measurement is made pointwise using a radiation source, such as an X-ray or nuclear radiation source of suitable intensity, type of radiation and energy, the intensity of the radiation, attenuated by the sheet of material or by the material being measured, is measured by an ionization chamber. For the case that a thickness profile is to be measured over a whole width of the sheet of material, the source of radiation and the ionization chamber can be fastened to a carrier, which then, embracing the sheet of material to be measured, is moved transversely over it. However, it is a disadvantage in this case that the thickness continues to be measured pointwise, which leads, particularly in the case of very rapidly moving sheets of material, as is usually the case with mill trains, to the fact that only a basically incomplete statement can be made concerning the thickness of the material over the width of the sheet. Admittedly, it is possible to interpolate the measured values over the whole width of the material arithmetically; however, such a procedure does not always provide the desired accuracy.
A similar result is achieved if a plurality of ionization chambers is disposed linearly next to one another and the radiation source is moved in a traversing framework relative to the ionization chambers and transversely to the direction of motion of the sheet of material to be measured. The use of a linear source of radiation and an ionization chamber, which can be moved relative to this source, is also known.
However, when material is moving quickly, control processes and, with that, adjustments of a manufacturing process can be realized only with difficulty. For these reasons, measuring devices were developed with which continuous measurements over the whole width of the material are possible.
Such an apparatus for measuring the thickness of flat profiles is known from DE 31 40 714 A1. In this case, above the flat profile to be measured, one or more punctiform sources of radiation are disposed, to which, in each case, a plurality of ionization chambers are disposed below the flat profile. Since the sources of radiation are constructed as point sources and the radiation is masked in a fan fashion, the ionization chambers, assigned in each case to a radiation source, are directed towards the radiation source. For this purpose, the ionization chambers are disposed in a collimator girder, which is provided with cylindrical collimator openings, which have axes aligned precisely with the source of radiation. However, it is not achieved with this arrangement that the radiation intensity, reaching each ionization chamber, is the same.
A similar apparatus is evident from DE 37 07 107 A1. In this case, the ionization chambers or the detectors, distributed over the surface of the material to be measured, are disposed in several rows. With this, an improvement in resolution can be achieved.
It is a common feature of all these apparatuses that either an expensive mechanical traversing system is required or a highly precise alignment of the ionization chambers or of the detectors must be assured in order to arrive at comparable measurement results. Moreover, when several ionization chambers are used, an appreciable technical effort is required in order to assure, as far as possible, the same parameters for all ionization chambers. Failing that, an expensive, constant compensation must be carried out, since gas fillings necessarily result in changes in the parameters.